Investigators have revealed that passengers on an Air China flight endured a gut wrenching 19,600 feet plummet because the plane’s co-pilot needed to get his tobacco fix.

Oxygen masks fell from the cabin ceiling as flight CA106 from Hong Kong to Dalian suffered the sharp loss of altitude on Tuesday. The plane started its unplanned descent about 30 minutes after taking off, and data from flight tracking website FlightAware shows it fell 4,200 (14,100 feet) in four and a half minutes.

Passengers were told the aircraft was in emergency descent due to cabin decompression. Once the plane reached an altitude of 3,000 meters (10,000 feet), it steadied and soon began to gain altitude again. Despite the terrifying incident the flight was able to complete its journey and landed safely in Dalian.

The Civil Aviation Administration (CAAC) has revealed on Chinese social media platform Weibo that the unnamed co-pilot accidentally caused a drop in oxygen levels in the plane while trying to turn off air recycling fans so the vapor from his electronic cigarette wouldn’t spread into the cabin.

Air China’s website says smoking, including e-cigarettes, is strictly prohibited on all its flights.

“After an investigation to verify the incident, the decision is to suspend the related crew from flying and terminate the contracts in accordance with the law. The crew members who are responsible for the incident have been seriously dealt with,” the airline said in a statement on Friday night.